With the rapid development of mobile terminal technology, such products as a tablet Personal Computer (PC), a mobile phone, a portable media player, a portable navigator and an electronic reader come into a lot of families to be widely applied. For these portable terminals, the unified use of a standard Universal Serial Bus (USB) interface for charging is an inevitable trend. The government of China and European Union have made some mandatory or voluntary laws and regulations, such as Technical Requirements and Testing Methods for Charger and Interface of Mobile Communication Handset. These regulations are significant for improving the charging convenience, reducing the consumption cost, saving social resources, reducing electronic waste, controlling the emission of greenhouse gas and the like.
For a standard USB host interface for which the charging voltage and charging current are 5V and 500 mA respectively, the charging power is only 2.5 W. To solve the problem of charging a battery of large capacity quickly, a special circuit, which may recognize the category of a power supply end, is often designed in a mobile terminal device. If it recognizes that a user adopts a power adapter instead of the USB host for charging, the charging current may be increased to about 1 A so that the charging power reaches about 5 W. Accordingly, the charging time is shortened and the user experience is improved. However, influenced by a contact resistance, the charging current of a standard USB connector cannot be increased optionally. For example, MINI-type and MICRO-type USB connectors often used by a terminal will heat up and become unreliable once the charging current exceeds 1 A. Therefore, the charging power can only reach about 5 W by means of the above scheme.
The charging power of about 5 W is still insufficient for some mobile terminals having a large-capacity battery. For example, through the above scheme, the battery of a large-size tablet PC cannot be fully charged within 2-3 hours since its capacity may exceed 6,000 mAh. A common solution for this is to design a special large-power charging interface on a terminal device. However, this may increase the additional cost and volume of the terminal and further decrease the universality of the terminal device and charger thereof, which is not favourable for economy and environmental protection.